Renewable energy resources are becoming an increasingly important part of the electricity generation portfolio. To encourage renewable energy deployment, many governments provide financial incentives and have established renewable energy portfolio standards. For example, California has mandated that 33% of all energy consumed in California will be from renewable energy by the year 2020. Wind and solar are two widely used renewable technologies but both are unpredictable and require additional spinning reserves be available on other parts of the grid. These standby reserves are typically large coal-fired or natural gas plants running inefficiently at part load. As the penetration of wind and solar increases, additional pressure will be placed on grid resources to maintain its reliability.
There are now many converters of renewable energy sources (primarily solar and wind) connected into most major grid networks. Improvements of power semiconductors and signal processors have led to a new generation of power converters and control strategies for these converters. Recently, depending on converter type, there have been hardware and software methods proposed and implemented for increasing converter efficiency. Even so, power from renewable energy remains expensive compared to non-renewable sources of power. Efficiency is still one of the most critical parameters of a grid connected system that is being pursued to make renewables more competitive.
Solar power is the generation of electricity from sunlight. This can be direct as with photovoltaics (“PV”), or indirect as with concentrating thermal solar power, where the sun's energy is focused to boil water for steam which is then used to generate power. A solar cell, or photovoltaic cell is a device that converts light into electric current using the photoelectric effect. Film photovoltaic cells can be made inexpensively in bulk and with conversion efficiencies in the range of about 15% to about 35%. Concentrating photovoltaics are another method of solar power. These systems employ sunlight concentrated onto photovoltaic surfaces for the purpose of electrical power production. Solar concentrators of all varieties may be used, which are often mounted on a solar tracker in order to keep the focal point upon the cell as the sun moves across the sky. Tracking can increase flat panel photovoltaic output by 20% in winter, and by 50% in summer.
The largest solar power plants, with outputs in the range of about 350 MW, are concentrating solar thermal plants, but recently multi-megawatt photovoltaic plants have been built with peak outputs in the range of about 40 to about 100 MW.
Modern wind turbines range from around 600 kW to about 7 MW of rated power. The power output of a turbine is a function of the cube of the wind speed, so as wind speed increases, power output increases dramatically. Areas where winds are stronger and more constant, such as offshore and high altitude sites, are preferred locations for wind farms. Typical capacity factors are 20-40%, with values at the upper end of the range in particularly favorable sites.
Globally, the long-term technical potential of wind energy is believed to be five times total current global energy production, or 40 times current electricity demand. This could require large amounts of land to be used for wind turbines, particularly in areas of higher wind resources. Offshore resources experience mean wind speeds of ˜90% greater than that of land, so offshore resources could contribute substantially more energy. This number could also increase with higher altitude ground-based or airborne wind turbines.
Renewable energy power plants such as solar farms, wind turbine farms and hydro turbine facilities provide an important source of secure, non-declining and carbon-free energy available to the power grid of a region. These renewable sources of energy are, however, not reliable as continuous sources of power to an electrical grid and therefore typically serve as supplementary sources of energy to the grid.
As renewables achieve higher levels of penetration, these low-availability or intermittently available renewable power generating facilities can reduce grid stability. In some instances, reliable fossil fuel generating plants, such as coal-fired power plants, expand their operations to create the spinning reserves that may be required when demand is high and the output from renewables is low.
Therefore, there remains a need for innovative methods to use other sources of prime power to provide fully available, continuous, reliable power, often called dispatchable power from power generating facilities that have a substantial fraction or all of their power generating capacity derived from intermittently available renewable energy sources.